Most hydroxy-terminated polybutadiene propellants exhibit mild to severe hardening or softening during high temperature aging. It has been determined that the described behavior is attributed to the reaction results of the bonding agent.
Numerous bonding agents have been employed in propellants to improve the binding efficiency and the physical properties. Certain bonding agents have been used to an advantage where it is known that the propellant would be subjected to either an extreme high temperature or low temperature environment. Typically, to determine long term aging behavior of a propellant, accelerated aging programs have been conducted in which the propellant is stored at elevated temperatures and physical properties over a wide temperature range are periodically measured.
Prior art bonding agents including MT4, HX752, and other types have been employed individually in aging studies to determine effects on propellant physical properties. A hydroxy-terminated propellant composition cured with a diisocyanate curing agent and containing MT4 as the bonding agent showed significant softening at high temperature (130.degree. F and 160.degree. F) aging. A propellant containing HX752 and cured with isophorone diisocyanate hardened severely under the same conditions.
Stress-strain curves depict significant changes in physical properties as evidenced by the shape of the curves. For example, propellants employing MT4 have flat or saddle shaped stress-strain curves at -40.degree. F. Propellants employing just HX752 give extremely regressive curves at -40.degree. F. This indicates rapid binder deterioration after reaching a critical stress value and results in poor low temperature properties.
Desirable would be a combination of bonding agents which results in greatly improved high temperature aging characteristics and in improved low temperature physical properties for the hydroxy-terminated polybutadiene propellant composition wherein used.
Therefore, an object of this invention is to provide a combination of bonding agents which greatly improves the high temperature aging characteristics of the hydroxy-terminated polybutadiene propellant compositions wherein used.
Another object of this invention is to provide a combination of bonding agents which results in improved low temperature physical properties of the hydroxy-terminated polybutadiene propellant composition wherein used.